Each projector displays an image, but the images are offset slightly. The projectors, meanwhile, are sending out light that is polarized. That means that at certain angles half the light is absorbed.
It's possible to see this effect with sunglasses; two polarized lenses. Put one in front of the other and start rotating it, and it's not possible to see through them when one is perpendicular to the other. In movie theaters, the 3-D glasses are polarized so that each eye only picks up one image at a time, giving the illusion of depth. Two projectors, though, can be cumbersome and expensive.
There are single projection methods, but those require even more moving parts, involving physical barriers akin to venetian blinds between the screen and the viewer. Called the parallax barrier method, the barriers limit which image the eye sees, creating a 3-D illusion.
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To fix this, the South Korean team, led by Byoungho Lee, professor at the School of Electrical Engineering at Seoul National University, used polarizers to stop the passage of light after it reflects off the screen rather than doing so at the projector.
The polarizer is a coating called called quarter-wave retarding film. It acts like the polarizers in two-projector systems, except instead of relying on two images, it splits up the single one coming off the screen to the eye. Basically, it moves the 3-D glasses to the screen, so the audience no longer has to wear them.
It will be a while before theaters use this, but it's been shown to work in at least two types of displays, and offers a path to cutting the costs (and the admission prices) of 3D movies.
via OpticsInfoBase, Optical Society